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O: Fachverband Oberflächenphysik

O 97: Ultrafast Electron Dynamics at Surfaces and Interfaces III

O 97.1: Vortrag

Freitag, 5. April 2019, 10:30–10:45, H16

The transient modification of a ZnO surface upon photo-excitation — •Lukas Gierster, Sesha Vempati, and Julia Stähler — Fritz-Haber-Institut Berlin, Dpt. Phys. Chem.

ZnO is intensely explored due to its great potential especially in the field of optoelectronics and (photo-) chemistry. However, it is still under debate how the properties of ZnO vary with photoexcitation [1]. Here, we use time- and angle resolved photoelectron spectroscopy to investigate the transient electronic properties of the ZnO (10-10) surface under resonant photoexcitation of the band gap. At low excitation densities, a small additional signal below the Fermi level is observed, consistent with the previously observed surface exciton formation [2]. Above the Mott density, within about 60 fs an intense spectral feature builds up which exhibits the characteristics of a thermalized electron gas. Likely, this new state results from bending the conduction band below the Fermi level by band gap renormalization: The ZnO surface becomes transiently metallic for hundreds of picoseconds. For all excitation densities a fraction of the pump-induced electronic population survives the inverse repetition rate of the laser (5 µs). The photostationary population must be stabilized by a positive localized charge. Potential candidates are defects and photogenerated hole polarons, as recently observed for ZnO (10-10) [1]. [1] H. Sezen et al, Nat. Comm. 6, 6901 (2015) [2] J.-C. Deinert et al., PRL 113, 057602 (2014)